Observations on normal EEG activity in different brain regions of the unrestrained swine

This methodological and electrophysiological study was undertaken with the aim of obtaining possibilities for evaluating objectively the ethical value of using high concentration CO₂ inhalation as pre-slaughter anaesthesia for swine. A technique for implantation of recording electrodes into the porcine neocortex and limbic system is described, as are results of EEG recording from these brain regions in the unrestrained animal. In addition to expected desynchronized activity in the awake animal, bilateral recordings from the frontal neocortex revealed the irregular occurrence of 'sleep spindles' during apparent non-REM sleep. The 'spindles' were sometimes unilateral, and when bilaterally simultaneous, the amplitude was often larger in one of the hemispheres. The EEG recorded from the dorsal hippocampus was characterized by theta-wave activity. The offering of food to the hungry animal, and subsequent brief feeding periods, were associated with marked accentuation of this theta activity. Recording via electrodes placed within the amygdaloid nuclear complex showed high-frequency activity irregularly interrupted by slow waves. However, if one of the 'amygdaloid' twin electrodes hit the ventral pole of the hippocampus, theta activity was dominant. Electrodes placed adjacent to the amygdaloid nuclei in the pyriform cortex exhibited a basic awake activity similar to that recorded from these nuclei. During apparent non-REM sleep, however, trains of slow waves were seen simultaneously with the appearance of 'sleep spindles' in the ipsilateral neocortex.